A cell is a basic unit constituting a battery. According to the electrolyte used, rechargeable cells can be generally classified into two types, namely, acid battery and alkaline battery. The electrolyte for the acid battery can be a water solution of sulfuric acid. One example of such acid cell is a lead-acid battery. The lead-acid battery has some disadvantages, such as bulky, heavy, causing pollution and slow redox reaction, and therefore has been replaced by lithium iron phosphate battery. An acid secondary cell can store electrical energy in the form of electric current, and will become damaged when the stored electric current (I) is fully discharged (i.e. when I=0).
The electrolyte for an alkaline secondary cell is mainly a water solution of potassium hydroxide. Some examples of such alkaline rechargeable cell are alkaline zinc-manganese battery, nickel-cadmium battery and nickel-hydrogen battery. The alkaline rechargeable cell can store electrical energy in the form of voltage and will become damaged when it is discharged to have a voltage of zero, i.e. when V=0. Generally, a battery is dead and can no longer discharge when it is discharged to have a voltage lower than 1.0V. The alkaline rechargeable cell must be charged with very small electric current and usually requires more than 24 hours to be fully charged. Further, the alkaline secondary cell can be recharged several times only and is therefore inconvenient for use. Moreover, the alkaline rechargeable cell tends to have a rising temperature when it is being charged and discharged.
All the electrical appliances that use secondary cells as a power source are internally provided with a battery management system (BMS). Normally, the battery management system has the function of measuring battery voltage to prevent or avoid battery over-discharge, over-charge, overheat, and other abnormal conditions. For an electrical device that uses multiple parallelly connected rechargeable cells as its power supply, the electrical device will stop supplying power when the battery management system thereof detects any one of the rechargeable cells has an insufficient voltage. Most consumers do not understand why the battery having electrical energy stored therein just could not discharge to even cause crash of the load. The sudden crash of the load tends to cause danger, particularly when a motor vehicle suddenly loses its power supply during moving.